In direct injection spark ignition engines, fuel is injected directly into an engine cylinder. Since the fuel is injected directly into the cylinder, it is possible to rapidly adjust engine torque. Such ability is advantageous for performing engine speed control, especially during idle operation. For example, sudden loading from ancillary devices such as the power steering pump, can be quickly compensated by adjusting fuel injection amount into the cylinder.
In direct injection spark ignition engines, ignition timing is generally tightly coupled to fuel injection timing. Therefore, prior approaches using ignition timing to control engine speed are not applicable. Also, when operating in a lean air/fuel mode, where the throttle is generally mostly open to reduce engine pumping work, the range of air/fuel ratios is limited by emission constraints or engine operability limits. For example, during stratified operation at idle conditions, there is a minimum air/fuel ratio limit due to emission constraints. Similarly, there is a maximum air/fuel ratio limit due to combustion stability limits. Since the engine is unthrottled, giving a relatively fixed airflow, idle speed control using only fuel injection amount has a disadvantage in that the range of control authority is limited.
Given limits of air/fuel operation, the throttle may be adjusted thereby moving the air/fuel ratio limits, giving more control authority to the fuel injection amount. However, this method suffers from the disadvantage that adjustments in the throttle cannot quickly change the cylinder air charge amount. Controlling flow entering the manifold cannot rapidly control cylinder charge due to manifold volume. For example, if the throttle is instantly closed, cylinder air charge does not instantly decrease to zero. The engine must pump down the air stored in the manifold, which takes a certain number of revolutions. Therefore, the cylinder air charge gradually decreases toward zero. Thus, by adjusting airflow entering the manifold to prevent air/fuel operation in undesirable air/fuel ranges, air/fuel operation will occur in the undesired ranges during a transient period.